This invention is directed to a charging circuit, and in particular to a charging circuit for efficiently charging a secondary battery, such as a Ni-Cd storage battery or the like.
Although secondary batteries, such as Ni-Cd storage batteries are rechargeable, such batteries do have a distinct battery life. Among the conditions that determine the battery life is the manner in which the battery is charged. Certain methods of charging a battery can damage the battery. For example, when a battery is continually charged by a high current over a short period of time, the useful life of the battery is considerably shortened, when compared to a battery charged over a considerably longer period of time at a lower current.
Nevertheless, although it is beneficial to charge a DC cell, or battery formed of a plurality of DC cells, at a low current over a long period of time, such charging does not permit charging of the battery to a maximum capacity. Thus, when an electrical device such as an electric shaver utilizes a rechargeable DC cell as a power source, it is necessary to charge the battery to its maximum capacity between uses, in order to permit same to fully effect driving of the electric shaver.
An approach to charging a DC cell to a sufficient capacity is disclosed in Japanese patent application No. 74-110741, for a Utility Model. In the Japanese patent, a charging device that effects charging with a relatively large charging current at an early stage, and reduces or eliminates the application of charging current to the DC cell at a latter stage is provided. Specifically, the power supply applies a charging current to the battery. A control circuit controls the amount of current applied to the battery to be charged, and a battery voltage detecting circuit detects the voltage of the charged battery. In such an arrangement, the battery voltage detecting circuit detects a predetermined voltage level when the battery is fully charged, whereafter the charge current controlling circuit immediately lowers the charging current applied. Nevertheless, such an arrangement is less than completely satisfactory for several reasons.
In order to effect a complete substantially 100% charging of the secondary battery, the charge current controlling circuit is operated when a reference voltage indicative of the battery being completely charged, referred to as the charge-completion voltage, is detected to thereby reduce the amount of charge current applied to the secondary battery to be charged. When the difference between the battery voltage just prior to the charge current control circuit being operated and the battery voltage at a considerable period of time after the charge control circuit is operated is greater than the variations in the reference voltage inherent in the charge current controling circuit, an even larger charging current is applied to the secondary battery for an additional short interval of time, even though the charge control circuit is intended to be operated to decrease the current applied to the battery. The settling or decrease in voltage of the battery is gradual over the considerable period of time thereby likely causing the above noted condition. Thus, the application of large charging current, once the battery has been charged to the charge completion voltage accelerates the deterioration of the battery. Although such a result can be avoided by selecting a charge-completion voltage at a level less than the completely charged level of the battery, the lowering of the reference level for operation of the charge control circuit will prevent substantially 100% charging of the battery.
Among the further factors that render the aforedescribed charging circuit less than completely satisfactory include the inability to control the amount of charge current applied to the secondary battery due to variations in the impedence of the charge control circuit, variations in the operating voltage V.sub.BE applied to the base electrode of the transistor included in charge controling circuit, and the temperature characteristics of the charge control circuit. All of these factors are likely to contribute to the overcharging of the battery, thereby accelerating the deterioration thereof.